The cell systems in which there has been rapid commercial development have four major components, i.e., an anode (e.g., Li), an electrolyte (e.g., SOCl.sub.2), an electrolyte salt (e.g., LiAlCl.sub.4) and a cathode (e.g., carbon). The cathode serves as an electronically conductive and catalytic surface on which the electrolyte can be electro-reduced while the electrolyte or liquid cathode is SOCl.sub.2 in which a salt, LiAlCl.sub.4, is dissolved in order to make the solution conductive and to allow mass transport of Li.sup.+ ions to the cathode. This lithium thionyl chloride cell has the highest energy density of commercially available primary cells. It has the desirable characteristics of good low and high temperature performance, excellent shelf life and a high power density in high rate designs. However, lithium has a melting point of 180.5.degree. C. and, consequently, lithium cells should not be subjected to high temperature, short circuit, high rates of discharge or high rates of charge, in order to avoid melting the lithium. Molten lithium in contact with SOCl.sub.2 creates a very hazardous and unstable condition and may initiate a cell explosion.
It has been shown that a vent is an effective means of preventing explosions or thermal runaways caused by shorting spirally wound Li/SOCl.sub.2. Further, it has also been shown that a safe venting pressure of probably less than 200 PSI is required to avoid explosions and thermal runawys in Li/SOCl.sub.2 cells. This low venting pressure results from the fact that SOCl.sub.2 electrolytes have an intrinsically lower vapor pressure, see FIG. 1, as compared to commercially available Li/SO.sub.2 cells. The electrolyte for a Li/SO.sub.2 cell has a characteristically higher vapor pressure and therefore Li/SO.sub.2 cells use vents which typically burst in the 450 PSI region.
A mechanical vent for an electrochemical cell with an electrolyte having a low vapor pressure should ideally exhibit the following criteria: low vent pressure, consistent vent pressure, relatively low cost, adaptable for mass production, occupy a small area and volume on the cell, no degradation of function with time and service, and significant expulsion of electrolyte from the cell. The simultaneous satisfaction of all these criteria is difficult to achieve in a vent. For example, prior art such as embossed and coined vents as disclosed in U.S. Pat. Nos. 4,105,133, 3,918,610 and 3,850,339 are not very effective for low pressure requirements and these vents also do not meet space and volume constraints. These vents, when applied to a Li/SOCl.sub.2 cell, allow the cell to attain too high a temperature before venting and, therefore, thermal runaway occurs and an explosion results.
Vent designs with a simple flat diaphragm are known to have inconsistent bursting pressures, while improved devices which provide for a piercing point near the diaphragm, as in U.S. Pat. No. 4,307,158 also have inconsistent operation. This inconsistency is due to variations in diaphragm material flexure, sensitivity to the sharpness of the point, slowness of operation and blocking of the hole in the diaphragm by the point tip. Vents of the flat coined variety commonly known as "Boiler Vents" are expensive and do not meet the low cost criteria.